Design and Development of Complex Phase Steels with Improved Combination of Strength and Stretch-Flangeability

Alexis Graux (1) , Sophie Cazottes (1) , David De Castro (2) , David San-Martín (2) , Carlos Capdevila (2) , Jose Maria Cabrera (3) , Sílvia Molas (4) , Sebastian Schreiber (5) , Djordje Mirković (6) , Frédéric Danoix (7) , Damien Fabrègue (1) , Michel Perez (1)

1 MATEIS - Matériaux, ingénierie et science [Villeurbanne]
2 CENIM - Centro Nacional de Investigaciones Metalúrgicas
3 Open University of Catalonia [Barcelona]
4 CTTC - Centre Tecnològic de Telecomunicacions de Catalunya = Telecommunications Technological Centre of Catalonia
5 ThyssenKrupp Steel Europe [Duisburg]
6 Salzgitter Mannesmann Forschung GmbH
7 GPM - Groupe de physique des matériaux

This study presents the design and development of a hot-rolled bainitic steel, presenting a good combination of strength and stretch-flangeability, for automotive applications. Ti, Nb, and Mo were added in the steel composition in order to control austenite grain sizes, enhance precipitation hardening, and promote the formation of bainite. This study focuses on the effect of process parameters on final microstructures and mechanical properties. These parameters are the finishing rolling temperature, which conditions the austenite microstructure before its decomposition, and the coiling temperature, which conditions the nature and morphology of the ferritic phases transformed. A preliminary study allowed to determine the austenite grain growth behavior during reheating, the recrystallization kinetics, and the continuous cooling transformation curves of the studied steel. Then, a first set of parameters was tested at a semi-industrial scale, which confirmed that the best elongation properties were obtained for homogeneous bainitic lath/granular microstructures, that can be produced by choosing a coiling temperature of 500 ∘ C . When choosing those parameters for the final industrial trial, the microstructure obtained consisted of a homogeneous lath/granular bainite mixture that presented a Ultimate Tensile Strength of 830 MPa and a Hole Expansion Ratio exceeding 70%.

Mots clés: metallurgy; complex phase steels; microalloyed steels; hot rolling; stretch-flangeability

Domaines: Matériaux

Alexis Graux, Sophie Cazottes, David De Castro, David San-Martín, Carlos Capdevila, et al.. Design and Development of Complex Phase Steels with Improved Combination of Strength and Stretch-Flangeability. Metals, 2020, 10 (6), pp.824. ⟨10.3390/met10060824⟩. ⟨hal-03365797⟩